1. Field of the Invention
The present invention relates to an improvement in the "solvent stretch" process for preparing a microporous film.
2. Description of the Prior Art
Williams et al U.S. Pat. No. 3,839,516 discloses a process, herein referred to as the "solvent stretch" process, for preparing microporous films from a suitable precursor film having at least two components, e.g., an amorphous component and a crystalline component. The precursor film is typically contacted with a swelling agent and longitudinally stretched greater than its original length while still in contact with the swelling agent and then, while the film is in the stretched state, the swelling agent is removed.
Notwithstanding the broad descriptive language of this patent, it has heretofore been generally assumed that multidirectional stretching, of the precursor film described therein, in the presence or absence of a swelling agent, would cause many processing difficulties such as tearing of the microporous film.
Porous films which are prepared from a process which includes a biaxial stretching step are well known in the art as illustrated by U.S. Pat. Nos. 2,823,421; 3,725,520; and 3,813,461. The processes described in these patents, however, do not employ a "solvent stretch" process as described herein and are therefore not concerned with alleviating the particular problems peculiar to such process.
Consequently, there has been a continuing search for a method of achieving an improvement in the balance of the mechanical properties of a "solvent stretched" microporous film, such as tensile strength and tear strength, which would result from biaxially stretching a precursor film employed therein. There has also been a continuing effort to improve the permeability of a microporous film prepared in accordance with the "solvent stretch" process. The present invention has been developed in response to this search.
It is an object of the present invention to enhance the permeability porosity, and surface area, and improve the balance of mechanical properties in both the machine and cross machine directions of a microporous film prepared in accordance with the "solvent stretch" process described herein.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.